Brushless DC (“BLDC”) motors are known. They include, among other things, a stator and a rotor. The stator is typically made from laminated steel stampings which are stacked to form a cylindrical shape with a central opening for receiving the rotor. The steel laminations in the stator may be slotted or slotless. A slotless stator has lower inductance and can therefore run at very high speeds. The absence of “teeth” that form the slots permit reduced requirements for the cogging torque, thereby making slotless BLDC motors appropriate for low speed use as well. Slotless BLDC motors may be more expensive than slotted BLDC motors, however, because more windings may be necessary to compensate for the larger air gap between the rotor and stator.
More specifically, many existing slotless motor designs include an outer casing or housing, a stator, a rotor assembled in some fashion with permanent magnets, axially fixed relative to the casing and stator so as to be rotatable within the central opening or bore of the stator, and windings provided with the stator, which energize and magnetize the stator in order to apply a torque to the permanent magnet members affixed to or comprising the rotor. The stator may consist of a hollow steel cylinder, constructed of a solid iron core, steel laminations with a circular cross-section stacked to make a cylinder (as indicated above), or concentric rings of amorphous ferroalloy tape assembled by rolling or successive layering. The windings responsible for the drive and magnetization of the stator are then typically wound onto the stator in one of two ways. In the first approach, the stator is constructed to have external protrusions which serve as arms around which a coil may be wound, placed at a specified series of angular positions around the exterior of the stator. In another approach, the stator is a plain cylinder, with no exterior or internal features beyond those required for interfacing the stator to other components. The windings are attached directly to the inner bore of the stator using a bobbin or adhesive.
Such slotless motors eliminate the preferential magnetic circuits present in normal slotted, armature-wound motors, and the cogging torques and slot losses typically found in permanent-magnet-rotor based motors. In theory, slotless motors should be able to achieve higher efficiencies over a greater range of operational conditions vs. a typical slotted stator motor design. Moreover, the simplified stator leads to much simpler, and therefore cheaper, manufacturing of the motor. The simplification of the field coil winding process also improves manufacturability.
Notwithstanding the foregoing advantages of slotless motors, conventional designs are still in need of improvement. The external protrusion design is effective, but creates some preferential magnetization directions through the diameter of the cylinder, which creates some “slot losses” and cogging torque. These designs also increase manufacturing difficulty by adding armatures of a sort back into the manufacturing and assembly process, negating many of the manufacturing benefits of slotless motors.
However, this design does have the advantage of allowing a very close tolerance within the bore, minimizing the air gap between the stator and rotor, maximizing the efficiency of the slotless motor design and giving such motors a greater amount of torque vs. size.
The internal coil winding slotless motor design has the opposite set of problems—the stator is extremely easy to design and manufacture vs. traditional slotted stators or external armature slotless stators, and the coils are much easier to wind. However, the inclusion of the coils on the interior of the stator requires the presence of a large air gap between the stator and rotor, greatly reducing efficiency and available power of this slotless design vs. traditional slotted motors by increasing the reluctance of the magnetic circuit formed between the magnetic elements present in the rotor and stator material. There are also obvious reliability and heating issues when considering a coil simply adhered to the wall of a stator, only millimeters away from a rapidly spinning rotor. There are alternative attachment methods; however, none eliminate the above efficiency decrease due to the increased air gap.
Thus, there is a definite need for a slotless BLDC motor/actuator design which is as easy to make as the internal coils designs, but retains the close tolerances and higher efficiencies of the external coils designs.